DAKAR/MONROVIA (Reuters) - The man being treated for Ebola in Texas showed no signs of fever or symptoms of the virus when he left Liberia for the United States via Brussels on Sept. 19, Liberian and U.S. authorities said on Wednesday.

The case is the first to be diagnosed outside West Africa during the current outbreak, raising the prospect that the worst epidemic of the deadly hemorrhagic fever on record could spread to nations beyond the region.

Liberian Information Minister Lewis Brown said the man, who has not been identified by officials and is now in serious condition in an isolation ward, "manifested no signs of fever or symptoms of the virus" when he boarded the plane to Brussels, which means he was not infectious when he left.

Neither Liberian nor U.S. officials have officially identified the patient. However, Gee Melish, who said he was a family friend, identified the man in Texas infected with Ebola as Thomas Eric Duncan.

In Monrovia, Dr. Tom Kenyon, a U.S. Centers for Disease Control official, said the case in Texas was evidence of the global nature of the health threat rather than a failure of the screening process at Liberia's airport.

New data released by the World Health Organization showed that more than 3,300 people have died as the disease has spread in Sierra Leone and Liberia since the first cases were confirmed in Guinea's remote southeast in March.

In a sign of the mounting international response to the crisis, 162 Cuban doctors and nurses are due to arrive in Sierra Leone on Thursday to help the response in the country, where British troops are already preparing new treatment facilities.

Belgium's health ministry said U.S. experts had advised Brussels that the man was indeed not displaying symptoms and so would not have been in a position to pass on the virus.

A spokesman said that Belgium therefore did not need to trace fellow passengers or crew of Brussels Airlines, one of a very few operators still flying to Guinea, Liberia and Sierra Leone.

Brown said Liberia had put in place "stringent screening measures" that were preventing Ebola from spreading via air travel and the checks are being regularly reviewed.

While Liberia, Guinea and Sierra Leone have failed to contain the virus, Senegal and Nigeria have located and isolated cases relatively quickly. U.S. officials say they are confident they can stop it from spreading further.

"What this incident demonstrates is the clear international dimension of this Ebola crisis. For months, the Liberian government has been stressing that this disease is not simply a Liberian or West African problem," Brown said in a statement.

"We also have every faith that the United States authorities will successfully contain this latest case so it remains an isolated incident."

The outbreak has led to border closures and some restrictions in travel to and from the worst affected countries. But experts say such moves do more harm than good by crippling economies and hampering the aid response while having limited impact on the spread of the disease.

In Sierra Leone, senior presidential advisor Sylvia Olayinka Blyden resigned, days after she accused the government of undercounting the number of deaths in the country.

Data from the WHO, based on figures from ministries of health, on Wednesday showed 710 dead in Guinea, 1,998 in Liberia and 622 in Sierra Leone.